1. Field of the Invention
The present invention relates to a threshold voltage (Vth) modulation image sensor and, more particularly, to a Vth modulation image sensor that permits pixel miniaturization.
2. Description of the Related Art
Image sensors that employ CCD are widely used. However, CCD image sensors have a complex pixel structure and require a process that is different from an ordinary CMOS process. It is therefore not easy to incorporate a CCD image sensor in the same chip as that of a peripheral signal processing circuit. On the other hand, as an image sensor that can be fabricated by means of a CMOS process, a CMOS image sensor has been proposed. A general CMOS image sensor comprises, in a single pixel, a plurality of transistors and a photoelectric conversion element such as a photodiode, and hence miniaturization of the pixel size is difficult.
As means for permitting this pixel miniaturization, a Vth modulation image sensor that uses Vth modulation of a MOS transistor according to the amount of light received has been proposed. For example, such an image sensor is disclosed by Japanese Patent Application Laid Open Nos. H11-195778, 2002-353433, and 2002-329856. In a Vth modulation CMOS sensor, electrical charge (carriers such as holes, electrons, and so forth) is generated by a photodiode that is formed in a well region common to a detection transistor, the charge is accumulated in the well region, and the variation in the threshold voltage of the detection MOS transistor that accompanies this electrical charge accumulation is then outputted as an image signal. Because each pixel is constituted by a single photodiode and a single transistor, the sensor is suited to pixel size miniaturization.
In addition, it has been proposed that the detection transistor be constituted by a ring-like gate electrode, a source region within the ring, and a drain region outside the ring to dispense with a pixel isolation region made of insulation material and suppress a leak current arising from the pixel isolation region. Such an arrangement is shown in FIG. 16 of Japanese Patent Application Laid Open No. 2002-329856, for example. This image sensor forms a potential pocket for the accumulation of electrical charge in a well region, establishes an efficient accumulation of photoelectric converted electrical charge in the potential pocket, and increases the variation in the threshold voltage of the detection transistor caused by the accumulated electrical charge, whereby the sensitivity of the sensor increases.